Not sure if these have been mentioned or not. I loaded a saved character on DL12 and sometimes when an enemy hits me I get the "monstrous rage" buff. Aether Oddities and the golems on this floor have both given me the buff. I have no skills that should/would proc such a buff. The mummies on DL12 seem way too weak. About 40 life, they die instantly. The Butcher Yam's flavor text is the same as the first floor potato's. Actually, all of the potato-type enemies have exactly the same flavor text. I've also noticed that you can "outrun" enemies when there are a large number of enemies taking damage at once. For example, set off some runes of exploding or fireballs then run in the opposite direction. You'll take several steps before all the damage can resolve and the monster will not have moved an inch. Butchery also procs on magic spells. Maybe intentional? It seems to only proc on the spell itself and never on dots or burn damage (can you tell I'm playing a prmothean mage?).
I think one of the diggle gods blessing has a proc for monstrous rage buff. most likly you have a diggle gods blessing active?
This is actually a bug caused by holding the WASD button down during a period where they are taking DoT damage (fire, acid, brimstone etc) and the monsters will not move and remain in the same spot. It also very commonly crashes your game so only thing to do is just slow down during those periods and wait for the game to catch up to prevent unwanted crashes.